The present invention relates to lead-acid cells, and, more particularly, to calcium-tin-silver lead-based alloys used for the positive grid alloys in such cells.
Sealed lead-acid cells (often termed xe2x80x9cVRLAxe2x80x9d cells, viz., valve-regulated lead-acid) are widely used in commerce today. As is known, sealed lead-acid cells utilize highly absorbent separators, and the necessary electrolyte is absorbed in the separators and plates. Accordingly, such cells may be used in any attitude without electrolyte spillage as would occur with a flooded electrolyte lead-acid battery. Such cells are normally sealed from the atmosphere by a valve designed to regulate the internal pressure within the cell so as to provide what is termed an effective xe2x80x9coxygen recombination cyclexe2x80x9d (hence the use of the terms xe2x80x9csealedxe2x80x9d and xe2x80x9cvalve-regulatedxe2x80x9d).
The advantages that are provided by sealed lead-acid cells in comparison to conventional, flooded lead-acid batteries are substantial and varied. Sealed lead-acid technology thus offers substantial benefits by eliminating maintenance (e.g., cell watering), expense (e.g., acid purchases), environmental (e.g., expensive waste treatment systems and air-borne acid mist) and safety (e.g., acid burns) concerns.
It is thus not surprising that sealed lead-acid cells are widely used in commerce today for various applications that have widely differing requirements. In one type of application, generally termed as stationary applications, lead-acid cells are used, for example, for load leveling, emergency lighting in commercial buildings, as standby power for cable television systems, and in uninterruptible power supplies. The uninterruptible power supply may be used to back up electronic equipment, such as, for example, telecommunication and computer systems, and even as a backup energy source for entire manufacturing plants. When the principal power supply to the electronic equipment has been cut off, such as during a power outage, the sealed cells (typically many electronically connected together) provide a source of reserve power to allow the telecommunication or computer system to remain operational until the principal power supply can be restored. The uninterruptible power supply also will accommodate short, or intermittent, losses in power, so that the function of the electronic equipment will not be impaired during a brief power outage.
In addition, there are many applications where sealed lead-acid cells are used in what are termed as motive power application. Sealed lead-acid cells are thus used as the power source for electric vehicles, fork-lift trucks, and the like.
The performance requirements for these two basic types of applications vary significantly. On the one hand, stationary applications are generally float applications, i.e., the cells are generally on float (i.e., an external voltage supply connected to the cells is held slightly above the cell potential to maintain charge), with an occasional need for a deep discharge when the main power source fails or is otherwise interrupted.
On the other hand, motive power applications require repetitive deep discharges, down to a 80% depth of discharge or even somewhat greater. Suitable cells must thus be capable of enduring repetitive charge-deep discharge-charge cycling regimes for up to 500 cycles or even more. Indeed, it would be desirable to provide cells capable of enduring from 1,000 to 2,000 cycles or so.
Developing grid alloys that adequately satisfy the diverse criteria for both stand-by and motive power applications has been largely unsuccessful. This lack of success has resulted even though substantial attention has been given to this issue by those working in this field.
This relative lack of success can perhaps best be appreciated when the principal criteria are considered because such criteria are stringent and are varied. These criteria must be satisfied, regardless of the type of application. In general, and by way of a summary, suitable alloys must be capable of being cast into satisfactory grids and must impart adequate mechanical properties to the grid. Still further, the alloys must impart satisfactory electrical performance to the VRLA cell in the intended application. Satisfactory alloys thus must impart the desired corrosion resistance, not result in thermal runaway (i.e., must not raise the tendency for the cell to lose water via gassing) and avoid premature capacity loss (sometimes referred to as xe2x80x9cPCLxe2x80x9d).
More particularly, and considering each of the criteria previously summarized, suitable alloys in the first instance must be capable of being cast into grids by the desired technique, i.e., the cast grids must be low in defects as is known (e.g., relative freedom from voids, tears, microcracks and the like). Such casting techniques range from conventional gravity casting (xe2x80x9cbook moldsxe2x80x9d or the like) to continuous processes using expanded metal techniques.
The resulting cast grids need to be strong enough to endure processing into plates and assembly into cells in conventionally used equipment. Even further, suitable grids must maintain satisfactory mechanical properties throughout the expected service life. Any substantial loss in the desired mechanical properties during service life can adversely impact upon the cell performance as will be more fully discussed hereinafter.
Considering now the electrochemical performance required, the grid alloy for the positive plates must yield a cell having adequate corrosion resistance. Yet, the use of a continuous direct casting process, desirable from the standpoint of economics, ostensibly can compromise corrosion resistance. Such continuous processes thus orient the grains in the grids, thereby making the intergranular path shorter and more susceptible to corrosion attack and to early failures.
Positive grid corrosion thus is a primary mode of failure of VRLA lead-acid cells. When positive grid corrosion occurs, this lowers the electrical conductivity of the cell itself. Cell failure occurs when the corrosion-induced decrease in the conductivity of the grid causes the discharge voltage to drop below a value acceptable for a particular application.
A second failure mechanism, also associated with grid corrosion, involves failure due to xe2x80x9cgrid growth.xe2x80x9d During the service life of a lead-acid cell, the positive grid corrodes; and the corrosion products form on the surface of the grid. In most cases, the corrosion products form at the grain boundaries and grid surface of the lead-acid where the corrosion process has penetrated the interior of the xe2x80x9cwiresxe2x80x9d of the grid. These corrosion products are generally much harder than the lead alloy forming the grid and are less dense. Due to the stresses created by these conditions, the grid alloy moves or grows to accommodate the bulky corrosion products. This physical displacement of the grid causes an increase in the length and/or width of the grid. The increase in size of the grid may be nonuniform. A corrosion-induced change in the dimension of the grid is generally called xe2x80x9cgrid growthxe2x80x9d (or sometimes xe2x80x9ccreepxe2x80x9d).
When grid growth occurs, the movement and expansion of the grid begins to break the electrical contact between the positive active material and the grid itself. This movement and expansion prevents the passage of electricity from some reaction sites to the grid and thereby lowers the electrical discharge capacity of the cell. As this grid growth continues, more of the positive active material becomes electrically isolated from the grid and the discharge capacity of the cell decays below that required for the particular application. The mechanical properties of the alloy thus are important to avoid undue creep during service life.
Still further, and importantly, the use of the alloys must not result in thermal runaway. VRLA cells must avoid conditions in service in which the temperature within the cell increases uncontrollably and irreversibly.
It has been hypothesized that excessive water loss resulting in cell dry-out is the driving mechanism for thermal runaway in VRLA cells. This water loss can be caused by hydrogen gassing at the negative electrode or oxygen gassing at the positive electrode through the electrolysis of water, or both.
As the water content and thus the cell saturation is reduced, the oxygen recombination efficiency is increased. Since this recombination reaction is highly exothermic, this tends to heat the cell. As the temperature rises, the cell tends to generate gas; and the recombination processes become even more efficient, thereby further increasing the cell temperature. In similar fashion, water loss increases the cell electrical resistance; and such increased cell resistance increases the cell temperature, thereby further increasing water loss. The cell is in thermal runaway.
Accordingly, to avoid alloys that will push cells into thermal runaway, the effect of the alloy and its constituents on gassing at both electrodes must be taken into consideration. As is well known, antimonial alloys have been considered necessary for positive grids where the cells are required in service to endure deep discharge-charge cycling regimes.
Yet, in general, although not exclusively, antimonial alloys cause thermal runaway in VRLA cells due to excessive gassing at both electrodes. Antimony thus leaches out from the positive grid as corrosion takes place, dissolving into the electrolyte, ultimately migrating to and xe2x80x9celectroplatingxe2x80x9d onto the negative electrode. These antimony sites on the negative electrode thus become preferential to hydrogen gassing. Additionally, the presence of antimony on the negative electrode increases the self-discharge and thereby heats the cell since the self-discharge current is also reflected in the float current.
Poisoning of the positive electrode, of course, also must be avoided. Undue gassing at the positive electrode can thus lead to thermal runaway.
Further, the alloys must maintain adequate contact for electrical conductance throughout the desired service life. Otherwise, the cell will experience what has been termed as xe2x80x9cpremature capacity lossxe2x80x9d (xe2x80x9cPCLxe2x80x9d).
PCL can also occur through loss of contact due to cracking of the corrosion layer or from a nonconductive film generated in the corrosion layer. Because of the complexity and the substantial potential adverse effects, this is a difficult criteria to achieve in combination with the other necessary criteria.
Lastly, it would be desirable to provide positive grid alloys capable of enduring deep discharge-charge cycling regimes. Satisfying this criteria would also allow use of such alloys for both motive power and stationary VRLA applications.
One singular exception to the lack of success in developing positive grid alloys for VRLA motive power and stationary applications is U.S. Pat. No. 4,401,730, issued to Joseph Szymborski et al., and assigned to the assignee of the present invention. The Szymborski ""730 patent thus discloses a sealed, deep-cycle, lead-acid cell including a cadmium-antimony lead-based alloy in the positive grid.
These alloys have satisfactory mechanical properties, i.e., good mechanical processability in cell assembly, high strength and toughness. Such cadmium-antimony lead-based alloys can successfully be used in sealed lead-acid cells, while avoiding thermal runaway and other problems often encountered when using antimony-containing alloys.
Although these alloys have been found to have exemplary properties, such alloys also have significant drawbacks. First, cadmium has been identified as a carcinogen. Special precautions must now be employed when preparing and handling cadmium-containing materials. Moreover, the presence of cadmium makes such positive plates difficult to dispose of after the useful service life of the lead-acid cell. All scrap must be segregated and shipped to a smelter that is permitted to recycle cadmium. Indeed, some countries currently will not allow transport of hazardous substances, like cadmium, across their borders. Accordingly, it would be desirable to provide an alloy for use in a positive plate in a lead-acid cell that does not require the inclusion of cadmium, yet would possess the many desirable characteristics of the cadmium-antimony lead-based alloys disclosed in the ""730 patent.
Indeed, while these cadmium-antimony alloys have been used commercially for years and despite considerable efforts to find other alloys that satisfy the diverse criteria, satisfactory alloys are yet to be developed. Some of this effort concerns the calcium-tin-silver lead-based alloy family. Yet, despite all this effort, satisfactory alloys have not been discovered.
Accordingly, there still exists a need for a lead-based alloy which can adequately satisfy the diverse requirements needed for making grids for positive plates used in sealed lead-acid cells for motive power and stationary applications while avoiding the use of cadmium.
Accordingly, it is an object of the present invention to provide a lead-based alloy for a positive plate for a lead-acid cell that does not employ cadmium as an alloying ingredient, yet possesses adequate characteristics to allow use for VRLA motive power and stationary applications.
It is an additional object of the invention to provide alloys cast into grids by conventionally used techniques and having satisfactory mechanical properties to allow use in conventional lead-acid processes and assembly.
Another object of this invention is to provide a positive grid alloy that is not overly susceptible to premature capacity loss of the cell.
Yet another object of this invention is to provide a positive grid alloy that can be used to achieve satisfactory cycle life for stand-by and motive power applications.
Other objects and advantages of the present invention can be seen from the following description of the invention.
In accordance with this invention, it has been discovered that highly desirable positive grid alloys, particularly for VRLA cells can be made using calcium-tin-silver lead-based alloys when the alloy composition is maintained within certain defined limits. Thus, it has been found that lead-based alloys having from about 0.02% to about 0.05% calcium, from about 1.5% to about 3.0% tin, and from about 0.01% to about 0.05% silver, the percentages being based upon the total weight of the alloy, possess highly desirable characteristics. Optionally, the alloys of this invention can include from about 0.003% to 0.03% by weight aluminum.
Indeed, the calcium-tin-silver alloys of this invention possess properties allowing use in VRLA cells for motive power and stationary applications.